Controlling magnetic interfaces using ordered surface alloys

We have investigated the growth and magnetic properties of Fe thin films on the clean W(100) surface and W(100)-M c(2 × 2) ($M=\mathrm{Cu}$, Ag, Au) surface alloy substrates. The influence of the interface on magnetism is assessed experimentally by studying sensitive threshold behavior in magnetic ordering using spin-polarized low-energy electron microscopy. The onset of ferromagnetic order that occurs with increasing film thickness at room temperature due to finite-sized scaling of the Curie temperature varies reproducibly among films on W(100) and the surface alloys. Magnetic moments and exchange coupling constants of the magnetic ground states are also determined theoretically for films with ideal interfaces by first-principles density functional theory calculations. These microscopic quantities are consistently enhanced in Fe films on the noble metal-induced surface alloys compared to their values in films on the clean W(100) surface. We attribute the systematic variation of magnetic onset observed experimentally to the competition between the intrinsically enhanced magnetic coupling and moments on the surface alloy substrates and several extrinsic factors that could suppress magnetic ordering, including intermixing, substrate and film roughness, and surface alloy disorder. Tendencies for intermixing are explored theoretically by determining the energy barrier for noble metal segregation. Despite these possible extrinsic effects, the results suggest that the use of the broad class of ordered surface alloys as alternative substrates may offer greater opportunities for manipulating thin film magnetism.

Figure 1

LEEM image integrated intensity vs film thickness for Fe growth on (a) the W(100) surface at 500 K, (b) the W(100) surface at 300 K, and (c) the W(100)-Au $c$(2 × 2) surface alloy at 300 K. The imaging energy was (a) 8.0 eV, (b) and (c) 1.0 eV. The Fe deposition rate is determined from the time interval between the start of deposition and the first oscillation peak during growth on W(100) at 500 K.

Figure 2

(a) LEEM image of the sample area on the W(100) surface that was studied in all experiments. (b)–(f) SPLEEM magnetic asymmetry images acquired during Fe film growth on W(100) at 300 K. Film thicknesses are (b) 2.78 ML (onset in upper step bunch region), (c) 2.83 ML (onset in lower step bunch region), (d) 2.90 (onset on flat terraces), (e) 3.00 ML, and (f) 3.50 ML. The imaging energy is 1.0 eV, and the image field of view is 13 μm.

Figure 3

SPLEEM magnetic asymmetry images acquired during Fe film growth on (a)–(d) the W(100)-Au $c$(2 × 2), (e) W(100)-Ag $c$(2 × 2), and (f) W(100)-Cu $c$(2 × 2) surface alloys at 300 K. Film thicknesses are (a) 2.90 ML, (b) 3.12 ML (onset), (c) 3.20 ML, (d) 3.72 ML, (e) 3.36 ML, and (f) 3.49 ML. The imaging energy is 1.0 eV, and the image field of view is 13 μm.

Figure 4

Variation of magnetic asymmetry with film thickness during multiple Fe deposition experiments on the clean W(100) surface (□), and Au- (◯), Ag- (△), and Cu- (⋄) induced surface alloy substrates at 300 K.

Figure 5

(a) The magnetic onsets determined at 300 K in multiple Fe deposition experiments on the clean W(100) surface (□) and W(100)-M c(2 × 2) surface alloys $\mathrm{[}M=\mathrm{Au}$ (◯), Ag (△), Cu (⋄)] are plotted with respect to Fe deposition rate. (b) The magnetic onsets determined at 300 K on the W(100)-Au $c$(2 × 2) surface alloy are plotted with respect to Au deposition rate used to prepare the surface alloy at 750 K (□), 800 K (◯), 810 K (△), and 860 K (⋄).

Figure 6

SPLEEM magnetic asymmetry images acquired during Au deposition on an 2.83 ML Fe film on the W(100) surface that exhibits weak magnetic ordering initially in (a) at step bunches (arrow). Au coverages are (a) 0 ML, (b) 0.1 ML, (c) 0.2 ML, and (d) 0.5 ML. The imaging energy is 1.0 eV, and the image field of view is 13 μm. (e) Variation of magnetic asymmetry with increasing Au coverage measured in the white box indicated in the images.

Figure 7

Calculated exchange interactions for (a) 2 ML Fe and (b) 3 ML Fe on different substrates. (c) Exchange coupling constants projected onto a [010] plane perpendicular to the substrate surface for Fe films on W(100)-M c(2 × 2) surface alloy (replace $M$ with W and set $J_{ij}^a=J_{ij}^b$ for Fe layers on the clean W(100) substrate). The substrate layer and second Fe layer are in the same vertical plane. Refer to the text for an explanation of the notation, $J_{ij}^{},J_{ij}^{a,b}$.

Figure 8

Calculated energetics for Au (■), Ag $\left(\text{.}\right)$, and Cu (▲) atom segregation from the substrate W(100)-M c(2 × 2) surface alloy layer to the 0.75 ML Fe film layer along the concerted exchange pathway shown in the projected [011] cross-sectional plane. The black, gray, and white filled balls represent W, Au/Ag/Cu, and Fe atoms, respectively.